The present invention relates to the restoration of engines and, more particularly, to the remanufacture of cast iron block engines using prefinished cylinder liners.
Within a reciprocating engine, the space between each piston and its cylinder wall is sealed by several piston rings. Each ring is resiliently urged in an outward radial direction so that sealing contact with the cylinder wall is maintained. Thus, during operation of such engines, sliding frictional contact between piston rings and cylinder walls wears the ring contact surfaces and cylinder walls. As wear progresses, compression is more difficult to maintain and, additionally, there is increased oil consumption and increased oil contamination from combustion products forced past the piston rings.
In general, replacement parts and restoration methods are available for large engines and engines which are required to be highly reliable, such as aircraft engines. Many diesel engines are designed and built with replaceable cylinders, and replacement cylinder and piston sets are manufactured and made available for overhauling diesel engines. The same is often true of aircraft engines, particularly large engines. Smaller aircraft engines are usually restored using remanufactured cylinders wherein the inner cylinder surface is plated to build up wall thickness and then machined to the desired dimensions and tolerances. One aspect of each of these types of restoration operations is that each is very expensive and can only be justified economically in commercial operations or in situations in which the cost of replacement of the entire engine is prohibitive.
In order to increase fuel economy by reducing overall vehicle weight, several automobile manufacturers have designed and produced engines wherein the cylinder block and heads, crankcase, and oil pan are formed of aluminum alloys. While such aluminum alloys perform adequately in a structural sense, they are too soft to stand up to high speed frictional loads and would wear quickly in such applications. To avoid this, other metals or materials are employed for components subject to friction, such as bearings. In particular, cast iron cylinder liners are employed for frictional sealing engagement with the piston rings. Such cylinder liners for aluminum block engines are referred to as "wet cylinder" liners because the engine coolant directly contacts the outer surfaces of such liners to carry away heat from engine combustion and piston friction.
Occasionally, during the operation of a piston engine, the piston rod can become separated from either the piston or the crankshaft. This is referred to as "throwing a rod" and can be very damaging to cylinder walls and to other components within the engine. The usual result is a gouging of the cylinder wall. If the gouge is not too deep, the cylinder can often be repaired by boring out the affected cylinder and the insertion of a replacement cylinder which is then bored and honed to the required dimensions within the block. Because it is necessary for all the cylinders of an engine to have the same volumetric displacement for engine balance purposes and since it is not economical to bore out and reline the remaining cylinders, the usual procedure is to redimension all the cylinders, including the repaired cylinder, to a greater radius than the original dimensions and to install oversized pistons. In older engines which were not computer controlled and wherein emission standards were not so strict, such repair methods were usually adequate. However, such relining of cylinders of modern engines which changes the displacement of the engine causes operational problems since the control computer and pollution controls are set up for controlling the operation of an engine with a given displacement.
Because conventional automobile engines with cast iron blocks are manufactured in mass quantities, it is currently not economical to restore the worn cylinders of an engine block to their original specifications. If an engine replacement is needed for an otherwise serviceable vehicle, it is currently more economical to install a new engine or an engine salvaged from a wrecked vehicle. Cast iron engine blocks with worn cylinders are usually discarded or recycled as scrap metal. This is a waste of the energy and resources which were expended in manufacturing the engine block.
In engines for which cylinder replacement is designed from the outset, the cylinder structures are relatively thick walled. This facilitates machining and honing of the inner cylinder surface since the cylinder structures are self-supporting and stiff enough that there is virtually no distortion of the cylinder wall during machining operations. This allows quick and accurate dimensioning and finishing of the inner cylinder surface. Such engines and replacement cylinders are also designed for relatively convenient replacement of worn cylinders without machining of the cylinder block. In the case of diesel engines, an upper lip of the cylinder engages a shoulder groove in the block while the lower end engages a similar shoulder. The cylinder is then held in place by the cylinder head. Adhesives are often used to seal between the upper rim of the cylinder and the block to prevent the entry of carbon therebetween would act as an insulator and interfere with heat transfer.
Conventional automobile engines with cast iron blocks are not designed for cylinder replacement. The cylinder structure is cast as an integral part of the cylinder block and machined to the required cylinder dimensions. When cylinder repair is required, relatively thin walled cylinder liners are used. In such a case, machining of the cylinder liner occurs with the liner located within the block. The liner wall is, thus, supported by the portion of the original cylinder wall which remains. Because of the relative thinness of the walls of such liners and the difficulty of accurately machining and finishing the inner surface thereof, it has heretofore been considered impractical to supply such liners in a prefinished condition, which would otherwise economize the restoration of such engine blocks to their original displacement specifications.